Before you draw a single mesh cell, a few hand numbers decide the physics, the mesh and the boundary conditions. This lesson works them out for natural gas entering a separator through an 8″ Sch 40 inlet nozzle.
What you will learn
- Mass flow, density and nozzle velocity from standard-volume flow
- Reynolds number: is the flow turbulent, and does it need a turbulence model?
- Mach number: can you use a constant-density, pressure-based solver?
- Wall shear, friction velocity and y+ to first-cell height
- Why y+ = 1 fails on a rough wall, and why wall functions at y+ = 50 are the right choice here
- Entrance length, and why you impose a developed inlet profile instead of modelling 11 m of pipe
Case data
25 MMSCFD natural gas · 60 barg · 40 °C · 8″ Sch 40 nozzle (ID 202.7 mm). Assumed: M 19.0, Z 0.88, k 1.27, viscosity 0.013 cP, steel roughness 0.046 mm.
Key numbers
| Quantity | Value |
|---|---|
| Mass flow | 6.57 kg/s |
| Gas density | 50.6 kg/m³ |
| Nozzle velocity | 4.02 m/s |
| Reynolds number | 3.18 × 10⁶ (turbulent → RANS, k-ω SST or k-ε) |
| Mach number | 0.010 (constant density OK) |
| Friction factor (Swamee-Jain) | 0.0145 |
| Wall shear / friction velocity | 1.48 Pa / 0.171 m/s |
| y+ = 1 wall distance | 1.5 µm (inside the 46 µm roughness) |
| y+ = 50 wall distance | 75 µm → first cell Δy = 0.15 mm |
| Entrance length | ≈ 53 D ≈ 10.8 m |
Takeaway: on this rough steel pipe, use wall functions with a roughness model, a 0.15 mm first cell, and a developed velocity profile at the inlet.
